This invention pertains to the processing of coal-derived liquids to remove ash and unconverted coal particulate solids. It pertains particularly to solids removal steps utilizing the continued precipitation and settling of such solids to produce liquid products having very low solids concentrations, by using a selected self-derived anti-solvent fraction at substantially ambient pressure.
It is known that gas turbines can burn residual oils but cannot tolerate ash contents in the oil greater than a few hundred parts per million, or alkali metal contents greater than a few parts per million. Also, sulfur in turbine fuel oils must be low enough to be within legal restrictions, and fuel bound nitrogen is a more serious problem than it is for boilers. Even for boilers the new source performance standard now require fuel ash contents below 0.03% for burning without electrostatic precipitation or bag filters for removing particulates for flue gases. Because the residual hydrocarbon material containing ash from coal liquefaction processes, such as the H-Coal.RTM. liquefaction process, is a low value product and because producing residual oil product consumes less hydrogen than producing distillates, there are strong economic incentives for upgrading such coal-derived residual oils to usable fuels by removing ash and the sulfur bound up in the ash to low concentrations.
Processes for removal of particulate solids from coal-derived liquids by the addition of a promotor or selected anti-solvent liquid to the coal-derived liquid stream containing unconverted coal and ash solids to produce precipitation and removal of such particulate solids have been previously disclosed in various patents. For example, U.S. Pat. No. 3,790,467 to Fiocco et al discloses a coal liquefaction process wherein solids are removed by precipitation from coal extract liquid by adding a solvent liquid which has been further hydrogenated. Similarly, U.S. Pat. No. 3,791,956 to Gorin et al discloses a coal liquefaction process in which a precipitating solvent which is an aliphatic or naphthenic hydrocarbon solvent having 75.degree.-200.degree. C. boiling range such as cyclo-hexane, n-decan, decalin, etc., is added to a stirred agglomeration zone which is maintained at 250.degree.-370.degree. C. temperature and 5-200 psig pressure. Such solvent liquid fractions do not normally occur in the liquid leaving Gorin's coal liquefaction zone, but may be recovered following subsequent hydrocracking steps, and produces a benzene-insoluble precipitated material. U.S. Pat. Nos. 3,852,182 and 3,856,675 to Sze et al disclose coal liquefaction processes wherein a promotor liquid having a characterization factor K of 9.75-11.0 is used to enhance solids precipitation and separation from a coal extract liquid. Furthermore, the K factor of the promotor liquid is at least 0.25 units higher than that of the liquefaction solvent. However, these processes require several processing steps and usually require the use of a petroleum-derived promotor liquid which must be recovered in high yield and in relatively pure form. They also require expensive pressurized settler vessels to achieve satisfactory solids removal from product liquid streams. An example of such settler vessel is disclosed in U.S. Pat. No. 3,962,008 to Sze. Also, U.S. Pat. No. 3,954,595 to Sze et al shows solids precipitation process for coal-derived liquid in which two gravity settlers in series are used for the removal of coal solids, and the overflow stream from each settler are joined together and passed to a recovery step.
Because of these problems, the use of more effective anti-solvent liquid fractions, which can be conveniently derived from the coal liquefaction process itself and which require simpler processing steps, have been sought.
Co-pending patent application Ser. No. 956,533, now abandoned by W. Volk et al discloses a process for solids precipitation from coal-derived liquids using an H-Coal.RTM. process naphtha fraction as an anti-solvent or precipitating agent to reduce the ash content of the process hydroclone underflow liquid to a concentration previously acceptable for boiler fuel (0.1% by weight). However, even lower concentrations of ash and sulfur in product liquids are desired, particularly for use as turbine fuels, as well as substantial removal of sulfur and nitrogen usually contained in the asphaltenes.